Re. Northern Tier, the few cyclists I spoke to (including the AC group) are all using one of the North Lakes and Lake Erie Connecter routes. I think that the SS Badger alternate (L. Michigan Ferry) is the most popular. They're certainly shorter than going south to Iowa.
I particularly enjoyed the ride on the Badger. From Luddington, I used local roads and trails near the w. shore of Michigan, plus the North Lakes to Monroeville, then the NT main to Clyde OH. I went off route to visit family, then reconnected to the NT Main just W. of Cleveland. I didn't think it was a bad route, in fact in the east 'burbs the roads are well set up for cycling with generous bike lanes. Follow the AC route through Erie, NOT the local "bike route", it's a glorified sidewalk and is unsafe IMHO.